The invention relates to a seismic acquisition method and system.
In seismic surveys with active sources, a source generates energy that propagates through the earth and is partially reflected back. The reflected energy is measured by receivers. The acquired data is then used to obtain information concerning the structure of the earth's sub-surface. Seismic surveys are very expensive. The surveys cost may be reduced by using several sources concurrently and thus decreasing the survey time. To make it possible, one should be able to recover the signal generated by an individual source from the combined signal generated by all the sources.
It is known from US patent application US2012/0014212 to fire an array of seismic sources in a distinctive loop of composite pulses where the returning wavefield is source separable based on the distinctive composite pulses thereby creating an identifiable loop of identifiable composite pulses so that two or more seismic acquisition systems can acquire seismic data concurrently. In a marine environment the peak energy delivered into the water may be less, which will reduce the irritation of seismic data acquisition to marine life.
A challenge in using seismic sources together resembles the cocktail party problem as formulated by E. C. Cherry in the article “Some experiments on the recognition of speech, with one and two ears”, published in the Journal of the Acoustic Society. Am. 25, 975-979, 1953. The cocktail party problem involves cross-talk problems generated by background noise generated by various people in a crowded room who simultaneously talk to each other. A listener who follows one of the speakers needs to separate his/her speech from voices of other speakers. The human brain can separate speeches. However this is a difficult problem for existing signal processing techniques.
Separation of controlled sources can be achieved by “tuning” the sources, so that the signal emitted by each source has distinct individual characteristics compared to signals emitted by other sources. For example, transmitters in radio communication emit signals in non-overlapping frequency ranges, which allows for a radio receiver to separate the signal sent by a selected transmitter and eliminate unwanted signals. This method is not applicable to seismic sources, because each source needs to fire all the frequencies within the seismic range (approximately from 0 to 100 Hz) to provide a good quality response. “Tuning” pulse-type sources which are used in marine acquisition is even more difficult, because their frequency content can hardly be changed.
Pulse-type sources arrays are distinguishable, when they emit pulses according to unique time ruler. The rulers are designed in such a way that they have proper de-ruler properties which makes each source array shot time ruler to become identifiable from the overall data. By using the proper de-ruler operator on the total received signal, the signature of a selected ruler can be recovered from the others.
The idea of using coded sequences of pulses to recover individual signals from simultaneously recorded sources is known in the industry. In particular, shooting schemes have been proposed that employ random firing patterns. The major challenge for application in the seismic domain is to design shooting sequences with good distinguishable properties to maximize the separation of the sources and minimize the emitted seismic energy. Coded sequences of pulses are also used in fiber-optic communication systems, where various sequences with good correlation properties (optical orthogonal codes) have been developed. These sequences however cannot be straightforwardly applied to seismic acquisition because the requirements to the seismic acquisition systems and to the fiber-optic communication systems are essentially different and mostly not based on maximizing the emitted energy but instead on the separation of data only.
It is observed that the mathematician Salomon Golomb devised a Golomb ruler with several marks (M) such that all pair-wise distances between marks are different and that single Golomb rulers have been applied for various technical applications outside the seismic acquisition domain.
Chinese patent application CN102904581 discloses a method to reduce storage complexity of a Low Density Parity Check (LDPC) code check matrix by constructing a check matrix of LDPC codes on the basis of a single Golomb Ruler.
Chinese patent application CN102412848 discloses a Quasi Cyclic-Low Density Parity Check (QC-LDPC) code construction method based on mode Golomb rulers to reduce search complexity.
US patent applications US201020026843 and US20110018484 disclose stepping motors with magnet pole pattern codes that may be based on a Golomb Ruler code.
US patent application US20090251256 discloses a coded linear magnet array which may have a polarity corresponding to a desired spatial force function that may be based on a Golomb Ruler spacing code.
Japanese patent application JP2011182067 discloses a speaker array wherein the speakers may be arranged at intervals that are proportional to a scale resolution of the shortest Golomb rulers.
Japanese patent application JP2005260743 discloses a microphone array wherein the microphones are arranged at distances proportional to a scale distance of a minimum Golomb Ruler.
It is furthermore observed that several non Golomb Ruler algorithms based on work by Salomon Golomb have been applied in the geophysical imaging domain.
International patent application WO2005096016 discloses a data compression method that may comprise a Golomb-Rice code. International patent application WO 02/091020 discloses a linear recursion formula defined by Salomon W. Golomb and International patent application WO01/71386 discloses an electroseismic waveform identified by S. Golomb.
It is furthermore observed that various quasi-random sequences, including a Golomb ruler, have been discussed in relation with shooting patterns of a single source, see Shaun Strong et al: “Numerical modelling of pseudo-random land seismic sources”, ASEG EXTENDED ABSTRACTS, vol. 2004, no. 1, 1 Jan. 2004 (2004 Jan. 1), page 1, XP055100932.
U.S. Pat. No. 6,906,981 B2 granted to Vaage Svein Toreif on 14 Jun. 2005 discloses a seismic acquisition method, wherein seismic pulses are emitted with varying time intervals in an unspecified quasi-random manner, see column 6, lines 22 and 23.
The term “a difference triangle set” was introduced by T. Klove: “Bounds on the size of optimal difference triangle sets”, IEEE Transactions on Information Theory, vol. IT-34, p. 355 (1988).
Difference triangle sets are used in communication theory, see J. P. Robinson and A. J. Bernstein: “A class of binary recurrent codes with limited error propagation”, IEEE Transactions on Information Theory, vol. 1. p. 106-113 (1989), and in combinatorial design, see C. J. Colbourn: “Difference triangle sets,” in The CRC Handbook of Combinatorial Designs, C. J. Colbourn and J. H. Dinitz, Eds. San Diego, Calif.: CRC, 1995, ch. IV. 14.
Difference triangle sets are equivalent to the so-called strict optical orthogonal codes that were proposed for fiber-optic code-division multiple-access (FO-CDMA) networks, see W. Chu and S. W. Golomb: “A note on the equivalence between strict optical orthogonal codes and difference triangle sets”, IEEE Transactions on Information Theory, vol. 49, p. 759-761 (2003).
Difference triangle sets can be constructed by splitting a single Golomb ruler in parts. Known methods to design single Golomb rulers are reviewed by K. Drakakis: “A review of the available construction methods for Golomb rulers”, Advances in Mathematics of Communications, vol. 3, p. 235-250 (2009). Methods to construct difference triangle set from Golomb rulers have been described by J. P. Robinson and A. J. Bernstein: “A class of binary recurrent codes with limited error propagation”, IEEE Transactions on Information Theory, vol. IT-13, p. 106-113 (1989) and by A. C. Ling: “Difference triangle sets from affine planes”, IEEE Transactions on Information Theory, vol. 48, p. 2399-2401 (2002). Other difference triangle set constructions are reviewed by J. B. Shearer: “Difference triangle set constructions”, IBM research report RC24623 (W0808-045), IBM Research Division (2008).
These prior art references do not teach or suggest that it is beneficial to emit seismic pulses with varying time intervals based on a plurality of Golomb rulers.
In marine seismic surveys seismic shots are generally emitted by submarine airgun arrays which emit seismic shots with time intervals of about 10 seconds to avoid cross-talk of acoustic reflections from consecutive shots.
Although Golomb rulers and different triangle sets are well known and they have been applied in various areas, specifically in wireless communication, it is not obvious beforehand that designs based on sets of several Golomb rulers can be beneficial in seismic data acquisition. In particular, it is not obvious that there exist such sets of Golomb rulers that meet the practical constraints imposed by actual seismic acquisition systems.
In conventional seismic acquisition, shots are done about every T=10 seconds. The shot duration can be extended in time. By applying the deconvolution procedure one compresses the signal generated by a source to a relatively narrow spike. The minimum spike time-width Δt is determined by the frequency contents of the signal. It is approximately equal to Δt=1/ν, where ν is the frequency of the highest harmonic in the signal temporal Fourier spectrum. In seismic acquisition, one deals with frequencies up to ν=100 Hz. Higher seismic frequencies cannot propagate through earth on sufficiently large distances due to attenuation. Hence, seismic pulses can be deconvolved to spikes with the width of about Δt=10 milliseconds. It means that sequences of seismic pulses can be designed using only those Golomb rulers whose lengths are not essentially larger than T/Δt=1000. Here, the length of a Golomb ruler is the number of unit intervals in this ruler.
There is a need for an improved seismic acquisition method and system wherein seismic reflections stemming from different seismic pulses and/or seismic sources can be accurately distinguished from each other.
Furthermore there is a need for an improved seismic acquisition method and system that provide shooting sequences with good distinguishable properties to maximize the separation of the sources and minimize the emitted seismic energy.